This invention relates to a control system for a pulse width modulated inverter-induction machine drive system and, more specifically, to a control system which utilizes feedback of an induction machine parameter for regulation of inverter thyristor switching intervals to achieve good inverter-induction machine drive system performance.
The ability to control both torque and speed of an induction machine, particularly in response to varying load conditions, is often desirable. Since the torque and speed of the induction machine are dependent on the amplitude and frequency, respectively, of the voltage supplied to the machine stator, machine torque and speed can be controlled by conditioning, that is, varying, the amplitude and frequency of induction machine stator voltage.
Conditioning of induction machine stator voltage is commonly achieved by use of an inverter coupled between a source of low frequency potential and the induction machine. The inverter typically comprises a plurality of pairs of serially-connected solid state switching devices, either high power transistors or thyristors, thyristors being preferred for high current operation, which switching device pairs are coupled in parallel across the low frequency potential source. The junction between switching devices of each inverter switching device pair is coupled to a respective phase of the induction machine. Rendering each of the switching devices of each switching device pair conductive in an appropriate sequence allows a voltage to be supplied to the machine stator, causing the induction machine to become excited. By controlling the frequency and duration of inverter solid state switching device conduction, induction machine stator voltage can be conditioned and hence machine torque and speed can be varied accordingly.
A common technique for controlling the conduction of inverter solid state switching devices is that of pulse width modulation whereby each of the solid state switching devices of each pair of solid state switching devices is alternately rendered conductive at a frequency more than twice the desired inverter output voltage frequency, producing an inverter output voltage waveform comprised of a series of positive and negative voltage pulses. The output voltage pulse width and hence, the inverter output voltage amplitude is controlled by adjusting the conduction duration of each inverter solid state switching device. The inverter output voltage frequency is determined by the solid state switching device conduction frequency.
Heretofore, analog feed-forward strategies such as the triangle interception method, described in the paper "Pulse Width Modulated Inverter Motor Drives With Improved Modulation" by Jacob Zubek et al. presented at the 1974 IEEE-IAS Annual Meeting in Pittsburgh, Pa. and published in the Conference Record in 1975, have been applied to inverter-induction machine drive systems to achieve pulse width modulation inverter operation. To achieve optimum induction-inverter machine drive performance using the triangle interception method, it is necessary to operate the inverter in several different modes in accordance with machine frequency. At low frequencies, the triangle reference waveform is unsynchronized with the sinusoidal reference waveform. As the fundamental machine frequency increases, the triangle reference waveform frequency must be synchronized to some multiple of three times the fundamental sinusoidal reference waveform frequency to avoid subharmonics in the inverter output voltage waveform. At still higher machine frequencies, when the magnitude ratio of the triangle reference waveform frequency to the sinusoidal reference waveform frequency becomes low because of inverter solid state switching device constraints, the lower order output voltage harmonics must be eliminated while still maintaining control of inverter output voltage. To obtain maximum inverter output voltage, pulse width modulation of inverter solid state switching devices is discontinued and each switching device is rendered conductive only once during each cycle of the inverter output voltage waveform. Consequently, analog implementation of the triangle interception method is complex because of the necessity to transition between each of the various modes without an objectionable jump in inverter output voltage.
In contrast, the present invention concerns a simple, digital apparatus which utilizes feedback control of an induction machine parameter, such as air gap flux, to achieve pulse width modulation of inverter solid state switching device conduction intervals to provide good inverter-induction machine drive performance.